The present invention relates to a method for preparing a silazane compound which is industrially important, for instance, as a starting material for preparing a variety of organopolysiloxanes and as a starting material for preparing silicon nitride and silicon carbide nitride.
The silazane compound can be prepared by reacting a halogenosilane with ammonia in an organic solvent. This method for preparing silazane compounds has been known for a long time and is disclosed in J. Am. Chem. Soc., 1948, 70, p. 3888.
According to this method, cyclotrisilazane is obtained as a mixture with other silazane compounds and accordingly the yield of cyclotrisilazane is very low. For instance, when dimethyldichlorosilane is reacted with ammonia, a mixture of hexamethyl cyclotrisilazane and octamethyl cyclotetrasilazane can be prepared.
However, octamethyl cyclotetrasilazane which is included in the resulting mixture has a high melting point in the order of 97.degree. C. and hence is solid at ordinary temperature. For this reason, it solidifies in a condenser through condensation during the usual distillation operation and it is correspondingly difficult to handle and to hence obtain it at a high purity. Under such circumstances, there has been desired to convert it into hexamethyl cyclotrisilazane which may be obtained at a high purity and is a liquid at ordinary temperature from the industrial standpoint.
As an example of a method for selectively preparing cyclotrisilazane, Japanese Patent Publication 63-58838 discloses a method which comprises heating the resulting cyclosilazanes other than cyclotrisilazane in the presence of hydrogen and a catalyst of a Group VIII metal. However, this method suffers from various problems. For instance, hydrogen which is a combustible gas is employed, the catalyst used is very expensive and the yield of the desired compound is low because of the formation of high molecular weight compounds.
In addition, Soviet Plast, (10), 1965 discloses a method for preparing an equilibrium mixture of octamethyl cyclotetrasilazane and hexamethyl cyclotrisilazane by heating octamethyl cyclotetrasilazane over a long time period in the presence of sulfuric acid or ammonium sulfate as a catalyst. However, in this method, the yield of hexamethyl cyclotrisilazane is likewise low because of the formation of polymeric compounds as by-products during establishing the equilibrium and the presence of octamethyl cyclotetrasilazane remaining in the system.
Alternatively, if hexamethyl cyclotrisilazane produced is distilled off outside the system (which is similar to the method of the present invention) in the presence of sulfuric acid or ammonium sulfate, a large amount of polymeric compounds are formed and accordingly the yield of the desired compound is also low as will be demonstrated in the Comparative Examples given below. This means that sulfuric acid or ammonium sulfate is not favorable for the industrial production of hexamethyl cyclotrisilazane.